A press-piercing rolling mill is a machine which can convert a square billet into an axially pierced, round intermediate blank. Such a machine basically comprises a rolling stand with two driven working rolls defining a circular gorge, a piercing mandrel carried at the end of a mandrel back-up bar which extends along the rolling and piercing axis from the exit side of the stand, and a pusher for thrusting a square billet between the rolls and against the piercing mandrel.
A basic condition for making round intermediate blanks which are pierced as exactly as possible along their longitudinal axis is that during the operation of the rolling mill, the mandrel back-up bar is firmly held on the rolling and piercing axis. For this purpose a plurality of pairs of constraining elements is used, these being supported in positions which are symmetrical relative to the rolling and piercing axis and being disposed in two lines parallel to this axis. During operation, each pair of constraining elements is maintained in engagement with the mandrel back-up bar, for example, by respective hydraulic cylinders. When during the course of the rolling operation, the pierced blank in its advancement along the mandrel back-up bar reaches the positions therealong where the latter is engaged by the pairs of constraining elements, these elements are operated to move them away from the bar so as not to obstruct the further advance of the blank.
For this purpose each constraining element is basically constituted by a vertical axis roller supported at one end of an arm the other end of which is rotatably mounted on a vertical pin carried by a slide. A hydraulic cylinder or similar actuator provides for the angular displacement of the arm about the corresponding vertical pin so as to position the roller in contact with the mandrel back-up bar or to space it therefrom.
The slide is movable on horizontal guides extending perpendicular to the rolling and piercing axis and its position is adjustable on these guides. In this manner, each pair of constraining elements may be adjustably positioned relative to the rolling and piercing axis to accommodate mandrel back-up bars of different transverse dimensions. Hydraulic or mechanical actuators are usually used to displace all the slides on their respective guides, these guides being mounted on the same base as that which supports the mandrel back-up bar and the bar thrust block carriage.
The inevitable existence of play in the couplings between each slide and its respective guide, and in the pivots coupling the arms to their corresponding slides and hydraulic actuators, results in the inexact positioning of the constraining elements relative to the rolling axis and, hence, a greater or lesser degree of eccentricity of the mandrel back-up bar relative to the rolling axis. As a result, the round intermediate blank produced has a piercing with even greater eccentricity and, in extreme cases, must be discarded.
To this technical disadvantage is then added the time taken by qualified personnel in suitable adjustment of the pairs of constraining elements.